1. Field of the Invention
The present invention relates to a storage device and a storing method, and more specifically, to a controlling method and an implementing method of a flash memory for enhancing reliability of a storing system using a flash memory.
2. Description of the Related Art
As a non-volatile semiconductor memory, there is a flash memory for which a deletion unit or a writing unit (functional capacity unit) is previously determined. The flash memory has a floating gate structure, which wraps a conductor such as a polysilicon in an oxide insulator such as an oxide silicon for storing information by the presence of the charge, which is in the floating gate. The flash memory may use a structure for reserving a charge in a particular place other than the floating gate structure.
When the charge is filled or drawn in the floating gate structure, the charge is moved while dielectric breakdown is performed on the oxide insulator, or the charge is moved with the tunnel effect of an oxide film. Consequently, the insulator is degraded. As such, the flash memory has a lifetime, with limited number of deletion and limited number of rewriting for each capacity of deletion unit.
A method for prolonging the lifetime of non-volatile semiconductor memory with a lifetime for each deletion unit capacity includes a method for evening up the number of deletion for each deletion unit over the entire chip and a method for enabling the non-volatile semiconductor memory to keep operating as a memory device without a problem even if some deletion units cannot be used by providing a substitute region for substituting the part of the deletion units whose lifetime has expired.
The methods can prolong the lifetime, however, cannot eliminate the lifetime. The flash memory has a lifetime and it dies if the lifetime has expired.
As there is a technique for prolonging a lifetime of the flash memory in the conventional art, it is impossible to eliminate the lifetime. As such, a storage system using a flash memory needs a mechanism for compensating a part of the flash memory whose lifetime has expired even when the lifetime has expired in that part. At that moment, a failure occurs that a part of the flash memory is impossible to be deleted or written. In such a case, new data merely cannot be written in the part and already stored data can be read out without any problem. Therefore, it is effective to substituting only a part, in which deleting or writing is impossible, with another means leaving the remainder kept being used.
As such, when a part of the flash memory has a failure, the part with the failure is substituted with another part, but it is required to manage which part has a failure and which part substitutes the part with the failure. Therefore, it is required to manage the logical address and the physical address for each deletion unit so as to manage which logical address is served by each deletion unit of the flash memory and which flash memory has corresponding data when the logical address is indicated.
The management of the logical address and the physical address is performed by the superior system, there is a problem in that the management table becomes too large, raising an implementing cost for a required memory or the like. There is a problem in that it takes much time in converting the logical address and the physical address and the performance is lowered when it is attempted to reduce the cost.
There is also a method for exchanging memory device including a deletion unit whose lifetime has expired in an external storage device in maintenance for keeping the capacity. The method has a problem in that it is ineffective to exchange a memory device when only a part of the memory device has become unavailable.